Linear accelerators (LINACs) are used to accelerate charged particles, such as electrons, protons, or ions, along a linear axis. In one example, a LINAC includes a number of resonant cavities that are disposed along an evacuated beamline. The resonant cavities are powered by a microwave source that provides oscillating electromagnetic fields within each resonant cavity. As the charged particles travel along the beamline, the oscillating electromagnetic fields within each resonant cavity are applied to the particles along acceleration gaps. In this manner, energy is transferred from the oscillating electromagnetic fields within the cavities to the charged particles so that the particles are accelerated.
The resonant cavities include an electromagnetic mode and, for charged particles travelling at sub-relativistic velocities, the mode across the resonant cavities is set to an appropriate phase velocity so that the oscillating electromagnetic fields are synchronized with the velocity of the charged particles travelling along the beamline. Otherwise, phase slippage may occur between the oscillating electromagnetic fields and the charged particles. Phase slippage produces inefficient transfer of energy from the oscillating electromagnetic fields to the charged particles and can inhibit net acceleration of charged particles.
LINACs compensate for phase slippage at sub-relativistic particle velocities by using shorter acceleration gaps and resonant cavity lengths (e.g., approximately ⅓ the oscillating electromagnetic field wavelength). Short acceleration gaps, however, are less efficient in transferring energy to the charged particles and more prone to instabilities, such as multipactor discharges and secondary electron emission. Furthermore, short acceleration gaps are more difficult to fabricate for very low velocity accelerators or may require greater electron injection energies. For example, the electron injection energy may be on the order of approximately 100 KeV or more. To generate high electron injection energies, LINACs use a high voltage system with high voltage insulation. These high voltage components increase the size and spacing requirements of the LINAC and constrain the design of the LINAC.